The present invention relates to a reactor containment vessel of a nuclear power plant and more particularly to improvements relating to a cooling system of the nuclear power plant to remove decay heat in the case of breaking of tubes or the like on an accident.
If an accident happens in a reactor by any possibility, it is necessrary to remove decay heat by means of cooling a reactor. In reactor containment vessels there have been proposed various kinds of cooling systems for removing the decay heat. For example, it is known that a suppression chamber and isolation condenser (emergency condenser) are respectively provided as a cooling system without using dynamic device like a pump. The reactor containment vessel is generally composed of a drywell and a wetwell, and the suppression chamber is provided in the wetwell to communicate with a drywell through a suppression vent pipe wherein pool water is charged. The isolation condenser is connected to a main steam line extending from the reactor pressure vessel to remove decay heat from steam introduced therein. The provision of such a system makes cooling operation of the nuclear power plant reliable.
If an accident happens in the reactor, for example, in the case of the loss of coolant accident (LOCA), the decay heat produces steam in the reactor pressure vessel. The steam is introduced into the isolation condenser through the main steam line. The introduced steam is condensed in the isolation condenser and returned to the reactor pressure vessel as condensate by gravity.
When the main steam line is broken in the drywell, the steam produced by the decay heat is released into the drywell from the reactor pressure vessel through the broken main steam line. Then non-condensable gas charged in the drywell is mixed with the steam and introduced into heat exchanger tubes of the isolation condenser. Such an invasion of non-condensable gas causes the heat-exchanging performance of the heat exchanger tubes to deteriorate.
To cope with these problems, in the above-mentioned conventional structure, the isolation condenser is provided with a non-condensable gas vent pipe which vents the non-condensable gas in the heat exchanger tubes to the suppression chamber.
However, if the pressure in the drywell becomes almost equal to that in the suppression chamber, the end of the non-condensable gas vent pipe is sealed by the suppression pool water and the non-condensable gas vent pipe stops venting the non-condensable gas to the suppression chamber.
However, if residual non-condensable gas exists even a little in the heat exchanger tubes, the existance of such non-condensable gas causes condensation heat transfer to deterioate. It is known by experiment that when the non-condensable gas is 10% at mass rate to the steam, the condensation heat transfer degrades at about 20% as compared with the case without non-condensable gas. Accordingly, to prevent the degradation of heat transfer characteristics in the isolation condenser, it is desirable that non-condensable gas should be excluded from the heat exchanger tubes as much as possible.